1. Field of the Invention
Embodiments of the invention relate generally to a method and apparatus compensating disturbance in a state control device. More particularly, embodiments of the invention relate to a method and apparatus reducing the effect of disturbance applied to a head driving unit in a track following control device of a hard disk drive (HDD).
This application claims the benefit of Korean Patent Application No. 10-2005-0055905, filed on Jun. 27, 2005, the subject matter of which is hereby incorporated by reference.
2. Description of the Related Art
A track following control device within a hard disk drive (HDD) is commonly adapted to locate (or position) a read or read/write head on the center of a target track. Disturbance (e.g., mechanical shock or vibration) applied to a driving unit associated with the head may cause a track following error. Accordingly, it is necessary to effectively eliminate or remedy such disturbance.
Figure (FIG.) 1 is a block diagram of an exemplary track following control device as implemented in a conventional HDD. Referring to FIG. 1, the track following control device is a type of state control device including a plant 104 and an estimator & controller 106. The estimator & controller 106 is adapted to estimate a current state for plant 104 and control the state of plant 104 based on the estimated state.
Plant 104 corresponds in the illustrated example to a head driving unit within the HDD. An output of plant 104 may include servo samples associated with the head, (i.e., position information associated with the head).
The estimated state provided by estimator & controller 106 includes position information related to the head driving unit, position and speed information related to the head, and corresponding servo samples associated with the head. Based on the estimated state information, estimator & controller 106 generates a reference control signal adapted to maintain (or achieve) a defined state indicated by a reference signal.
In FIG. 1, disturbance signal “d” indicates the nature of disturbance to be compensated (e.g., amplitude and duration of the disturbance), and state signal “y” indicates a current state (e.g., position) of a controlled object, such as a head and/or head driving unit in the illustrated example.
Disturbance signal “d” may be frequency related. In such circumstances, it is particularly important to eliminate low frequency disturbance, (i.e., disturbances having a frequency less than a defined crossover frequency for plant 104).
A gain value associated with estimator & controller 106 may be controlled (e.g., increased or decreased) to effectively compensate for the disturbance indicated by disturbance signal “d”. As the gain of estimator & controller 106 increases, the compensation effect on low frequency disturbances may improve, but the effect of high frequency disturbance, (i.e., disturbances having a frequency greater than the crossover frequency of the plant 104) may actually suffer.
FIG. 2 is a graph illustrating a relationship between the gain of estimator & controller 106 and gain/frequency characteristics of an exemplary track following control device, like the one shown in FIG. 1. Relationships between gain and gain/frequency characteristics for four separate cases are shown in FIG. 2. Referring to FIG. 2, as the gain of estimator & controller 106 increases, a crossover frequency shifts to a higher value on the gain/frequency characteristic curve. Also, as the gain of estimator & controller 106 increases, gain in a frequency range less than the crossover frequency decreases, while gain in a frequency range greater than the crossover frequency increases. These results are referred to as the “waterbed effect”.
Referring still to FIG. 2, an ideal disturbance compensation outcome would be: (1) low frequency disturbances would be eliminated, (2) any shift of a crossover frequency would be modest, and (3) high frequency disturbances would not be increased.